This invention relates broadly to the pyrotechnic generation of mixtures of gases, and in particular to the generation of a breathable mixture of oxygen and nitrogen useful as a substitute for air. It is highly useful as a compact safety device for providing a breathable mixture for emergency use in smoke or other noxious or toxic environments. It is particularly advantageous where the proximity or threat of fire precludes the use of pure oxygen, and it provides a breathable mixture for a sufficiently long period, on the order of fifteen minutes or more, to substantially improve the user's chances of escape.
Pyrotechnic charges, commonly referred to as candles, have long been available for generating single gases for a variety of purposes. For example, oxygen candles, which generally employ sodium chlorate or potassium perchlorate, are commercially available for emergency use where the air supply is limited or contaminated. These devices generally supply the oxygen for breathing through a mask. However, such devices generate substantially pure oxygen which cannot safely be used in the presence of fire or imminent explosion.
There also exist devices employing multiple pyrotechnic compositions for generating mixtures of gases over fleetingly brief time periods--so brief that matching the reaction rates of the respective compositions and hence the rates at which they generate the components of the resultant mixture is not a problem. For example, U.S. Pat. No. 3,797,854 issued Mar. 19, 1974, to Rocket Research Corporation, discloses an apparatus for generating a breathable mixture of nitrogen and oxygen to inflate an air cushion in an automotive crash restraint system. The nitrogen and oxygen are generated by separate compositions, the ignition of which are synchronized by allowing the nitrogen-producing reaction to initiate the oxygen-producing reaction. Both reactions proceed at their own rates to generate large volumes of gas in a small fraction of a second so as to rapidly inflate the air cushion. Because both compositions must react completely in a very brief time period, only the ratio of the total volumes of the two gases need be controlled, and not the respective reaction rates.
It is important for breathing purposes that the average ratio between the oxygen and nitrogen produced be relatively constant over periods of, say, 15-30 seconds. Although some variation in the reaction rates may be tolerated for brief periods of time, significant departures from the average rates over longer time periods would produce atmospheres either too lacking in oxygen to be breathable, or so rich in oxygen as to create a significant fire and explosion hazard.
It has not been possible with known pyrotechnic compositions for generating these two gases to provide separate oxygen and nitrogen candles capable of producing breathable air in accordance with those criteria and in a practical manner. Although control of burn rates becomes easier as the rates themselves are increased, the use of separate candles operating at relatively high burn rates would require such a large volume of propellant as to be wholly impractical, and would generate unnecessarily large volumes of gas. If the oxygen and nitrogen generating compositions are intimately mixed in order to match their burning rates, in an attempt to achieve lower rates, unstable and potentially explosive compounds may result.
In order to provide a workable and practical gas generator for these purposes, the burn rates of the respective candles should be no more than a few inches per minute, preferably between about one-half and one inch per minute. Although conventional oxygen candles can be made to sustain combustion at these relatively low rates, no nitrogen generating composition has been found which is capable of sustaining combustion at under several times such rates on a reliable basis. For that reason, all practical emergency sources of breathable air presently in commercial or military use employ cannisters of compressed air, or compressed nitrogen and oxygen, the ratio of which is controlled by valving upon their release from storage. The relatively large volumes required to store such gases, even under pressure, are such that available units are generally limited to about five minutes of use.